1. Field of the Invention
The present disclosure relates to a backlight unit, and more particularly, a backlight unit providing uniform brightness.
2. Discussion of the Related Art
With development of information technologies, flat panel display (FPD) devices have been further spotlighted as visual information delivery media. To secure further increased competitiveness, the flat panel display devices have been needed to have low power consumption, a thin profile, light weight and high quality. Liquid crystal display (LCD) devices are representative flat panel display devices and display images using optical anisotropy of liquid crystal. The liquid crystal display devices have advantages of a thin profile, small size, lower power consumption, and high quality.
A liquid crystal display device displays images by individually providing picture information to pixels, which are arranged in a matrix form, and controlling transmittance of a liquid crystal layer corresponding to each of the pixels. Therefore, the liquid crystal display device includes a liquid crystal panel, in which pixels as a unit for producing an image are arranged in a matrix form, and a driving unit for driving the liquid crystal panel. In addition, since the liquid crystal panel is not self-luminous, the liquid crystal panel needs an additional lighting control means such that a difference in transmittance is revealed to the outside. A backlight unit is disposed at a rear surface of the liquid crystal panel and is used as a lighting control means.
In general, backlight units are classified into a side light type or a direct type according to a position of a light source with respect to a liquid crystal panel. In side light type backlight units, a light guide plate is disposed under the liquid crystal panel, and one or a pair of lamps, as a light source, are disposed at one side or at each of two sides of the light guide plate. Light from the lamps is refracted and reflected by the light guide plate to be indirectly provided to the liquid crystal panel. In direct type backlight units, a plurality of lamps is disposed directly under the liquid crystal panel, and light from the lamps is directly provided to the liquid crystal panel.
Backlight units include cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), and light emitting diodes (LEDs) as a light source. Among these, LEDs, which do not include toxic mercury (Hg), have been widely used due to their small sizes, low power consumption, and high reliability.
Therefore, a liquid crystal display device may include a liquid crystal panel and an LED backlight unit at a rear surface of the liquid crystal panel.
LEDs include red, green and blue LEDs emitting red, green and blue light, respectively, and are arranged with a predetermined interval between adjacent two thereof.
White light is obtained by lightening the LEDs at the same time and mixing colors. In large-sized liquid crystal display devices, to decrease power consumption, two or ten LEDs constitute a cluster or an array and are repeatedly arranged on a printed circuit board.
The LEDs are driven by a backlight driving circuit. The backlight driving circuit includes a controlling circuit unit, a sensing unit and a plurality of circuit units.
The controlling circuit unit of the backlight driving circuit will be described with reference to accompanying drawings.
FIG. 1 is a circuit diagram of illustrating a controlling circuit unit according to the related art, and FIG. 2 is a view of illustrating a constant current and a sensing current according to time.
In FIG. 1, the controlling circuit unit includes an operational amplifier (OP-AMP) 10, a thin film transistor 15 and a current sensor 18. The operational amplifier 10 maintains a constant current flowing through LEDs.
The thin film transistor 15 includes a gate electrode, a source electrode, and a drain electrode. The gate electrode of the thin film transistor 15 is connected to an output terminal of the operational amplifier 10. The drain electrode of the thin film transistor 15 is connected to the current sensor 18. The source electrode of the thin film transistor 15 is connected to an LED array 19, which includes a plurality of LEDs.
A constant voltage Vref and a constant current Iref, which are supplied from a printed circuit board (not shown), are inputted to a first input terminal of the operational amplifier 10, and a sensing voltage Vsen and a sensing current Isen, which are supplied from an output terminal of the current sensor 18, are inputted to a second input terminal of the operational amplifier 10.
The operational amplifier 10 compares the sensing current Isen with the constant current Iref and adjusts the thin film transistor 15 connected to an output terminal of the operational amplifier 10 such that the sensing voltage Vsen has the same value as the constant voltage Vref.
By performing the processes, the controlling circuit unit controls a luminous current ILED flowing through the LED array 19, and the LED array 19 constantly emits light.
By the way, an offset current Ioffset may be caused in the operational amplifier 10 due to a process variation. The amount of currents flowing through the LED array 19 may be decreased or increased by the offset current Ioffset. Thus, there is a problem that it is difficult to control the brightness of the backlight unit.
More particularly, the luminous current ILED associated with the brightness of the backlight unit is obtained by comparing the sum of the offset current Ioffset of the operational amplifier 10 and the sensing current Isen with the constant current Iref and is the same as the sensing current Isen that actually flows through the LED array 19.
Therefore, the luminous current ILED, which actually flows through the LED array 19, varies according to the offset current Ioffset.
The operational amplifier 10 is manufactured as a type of an integrated circuit (IC). A plurality of operational amplifiers is built in an integrated circuit, and an output of each of the plurality of operational amplifiers forms a channel.
At this time, a value of the offset current Ioffset at a channel may be different from that at another channel.
With reference to FIG. 2, since the value of the offset current Ioffset at a first channel Ch1 is different from the value of the offset current Ioffset at a second channel Ch2, a luminous current value 22 is smaller than a constant current value 20 by a first difference G1 at the first channel Ch1, and a luminous current value 24 is larger than the constant current value 20 by a second difference G2 at the second channel Ch2.
More specifically, the offset current Ioffset of each operational amplifier 10 is added to the sensing current Isen outputted from the output terminal of the current sensor 18 and is compared with the constant current value 20.
For example, when the constant current value 20 at the first channel Ch1 is 20 mA, if the luminous current value 22 flowing through the LED array 19 and the current sensor 18 connected to the first channel Ch1 is 19 mA and the value of the offset current Ioffset is 1 mA, 20 mA, which is the sum of the value of the offset current Ioffset and the luminous current value 22, is inputted to the operational amplifier 10.
Additionally, when the constant current value 20 at the second channel Ch2 is 20 mA, if the luminous current value 24 flowing through the LED array 19 and the current sensor 18 connected to the second channel Ch2 is 21 mA and the value of the offset current Ioffset is −1 mA, 20 mA, which is the sum of the value of the offset current Ioffset and the luminous current value 24, is inputted to the operational amplifier 10.
Accordingly, since 20 mA, which is the sum of the value of the offset current Ioffset and the luminous current 22 or 24, is inputted to the operational amplifier 10, the operational amplifier 10 judges that a current having the same value as the constant current Iref flows through the LED array 19 and the current sensor 18 and does not change an ON current of the thin film transistor 15. Consequently, even though the current actually flowing through the LED array 19 has a different value from the value of the constant current Iref, the current actually flowing through the LED array 19 is not compensated, and brightness is different at each channel.
Like this, the current having a different value from the value of the constant current Iref is applied to the backlight unit because of the offset current Ioffset, and there is a problem that the backlight unit cannot produce desirable brightness.